Growth and mortality of endangered land crabs (Cardisoma guanhumi) assessed through tagging with PITs and novel bootstrapped methods

TL;DR

This study introduces bootstrapped tagging and length-frequency analyses to assess growth and mortality in endangered land crabs, revealing slow growth, high mortality, and the superiority of new methods over traditional approaches.

Contribution

It is the first to combine bootstrapped tagging analysis with bootstrapped length-frequency analysis for land crabs, providing more accurate estimates and highlighting methodological limitations.

Findings

Crabs exhibit very slow growth with Linf far above Lmax.

Total mortality rate Z is approximately 2.18 per year.

Bootstrapped tagging analysis is 2.2 to 3 times more precise than traditional length-based methods.

Abstract

The land crab Cardisoma guanhumi Latreille, 1828 is harvested in several countries in Latin America, and a critically endangered species. This is the first study to conduct bootstrapped tagging analysis (BTA) together with bootstrapped length-frequency analyses (BLFA). Crabs were sampled monthly in a mangrove patch at Itamaraca Island (Brazil), over 12 months, and marked with PIT tags. Both methods (BTA and BLFA) indicate very slow growth and Linf far above Lmax. BTA estimates were K = 0.12 y-1 (95% CI: 0.024 to 0.26 y-1), Linf = 118 mm (95% CI: 81 to 363 mm), Phi' = 1.23 log10(cm y-1) (95% CI: 0.86 to 1.36 log10(cm y-1)). Seasonality in growth was significant (p = 0.006, 95% CI for C: 0.15 to 0.93, median: C = 0.56). Pairs of K and Linf always followed narrow Phi' isopleths. Total mortality was Z = 2.18 y-1 (95% CI = 1.7 to 4.5 y-1). Slow growth and a very high Z/K ratio highlight the need for protective measures. BTA results were 2.2 to 3 times more precise than BLFA. Traditional length-based methods produced grossly biased results, indicating the urgent need for new, robust approaches and a critical reevaluation of long-standing methods and paradigms.